1. Field of the Invention
The present relates to an activating catalytic solution for electroless plating and a method for electroless plating using the same.
2. Description of the Related Art
Electroless plating has been often employed for forming conductive patterns on a substrate, such as alumina, dielectric ceramic, polyimide glass, glass-epoxy, ferrite, etc., of electronic parts, such as high-frequency coils, dielectric filters, capacitors, hybrid-ICs, etc. The electroless plating requires use of an activating catalytic solution to activate the surfaces of the substrate.
An activating catalytic solution is applied on a substrate to form a photo-sensitive film. When the photo-sensitive film formed from an activating catalytic solution is radiated with ultra-violet rays or laser rays, the radiated region of the photo-sensitive film is selectively activated so that only the radiated region can be plated. Therefore, it is possible to form conductive films having fine patterns by using such an activating catalytic solution and photolithographic technology.
Conventionally, palladium acetylacetonate dissolved into an organic solvent such as chloroform has been used as an activating catalytic solution. Such activating catalytic solution using palladium acetylacetonate requires use of a great amount of an organic solvent for cleaning the substrate during the development process. However, it is not preferable to consume that much organic solvent in view of economic and environmental reasons. If not used in excess, the unnecessary portion of the photo-sensitive film formed by the activating catalytic solution is likely to remain on the substrate due to imperfect cleaning. This would causes plating metal to be deposited on such unnecessary portions, which results in poor resolution of metal patterns formed by the following electroless plating.
In order to solve the problem, Japanese Laid-open Patent Application No. 7-33601 discloses a hydrophilic activating catalytic solution including an oxalate such as iron oxalate, a palladium salt such as palladium chlorate and an alkaline solution.
According the Japanese Laid-open Patent Application, the activating catalytic solution is applied on the substrate surface by a spin-coat method or the like to form a photo-sensitive film. The photo-sensitive film is exposed to light such as laser rays, thereby depositing a palladium catalyst on the substrate. The photo-sensitive film may be exposed through a photomask for selective deposit of the palladium catalyst. In this case, unexposed regions of the photo-sensitive film are removed by water or water-based liquid. After dipping the substrate on which palladium catalyst is deposited in the electroless plating bath, an electroless plating film is formed on the substrate with the help of the palladium catalyst as an activating catalyst.
According to this technology, it is unnecessary to use any organic solvent during the development process since the activating catalytic solution is hydrophilic. Therefore, it is possible to eliminate use of organic solvents during the processes from preparation of the activating catalytic solvent to electroless plating. In addition, the unnecessary portion of the photo-sensitive film can be removed using a large amount of water. However, the conventional activating catalytic solution has the following problems.
Specifically, palladium and oxalate in the activating solution form palladium oxalate which has a low solubility in an alkaline solution and it is difficult to improve the solubility. In particular, iron oxalate employed in the Japanese publication has a distinct disadvantage in this regard. This causes a problem that the photo-sensitive film formed by using the catalytic solution has a low sensitivity and results in a low contrast between the exposed portion and unexposed portion. To cope with the problem, it may require a long duration of exposure, which the increases production cost and decreases the deposition efficiency of the plating metal.
Further, when the unnecessary portion of the exposed photo-sensitive film is removed with water, the iron oxalate is hydrolyzed to form hydroxides including the palladium catalyst. The hydroxide is fixed on the substrate and is not removed by water. This means that the unexposed portion of the photo-sensitive film is also activated and that the electroless plating metal is deposited on the unexposed portion, thereby reducing reliability of insulating properties between conductive patterns. This causes the necessity of removing the hydroxide by dipping the substrate in a sodium hydroxide solution of pH 14 or greater or to enlarge the space between the conductive patterns, which increases the manufacturing steps or sacrifices the ability make fine patterns.
In view of the foregoing reasons, there arises a demand for an activating catalytic solution which can solve the above-mentioned problems and enable deposit of a palladium catalyst by short time exposure, as well as removing remained photo-sensitive film effectively by the washing process.